1. Field of the Invention
The present invention relates to a lateral conductivity modulated MOSFET wherein a drain, a source, and a gate are formed on one surface of a semiconductor wafer.
2. Description of the Related Art
A lateral conductivity modulated MOSFET which utilizes conductivity modulation caused by electrons and holes accumulated in a drift layer is disclosed in M. Darwish et al. "Lateral Resurfed COMFET." Electronics Letters. 7th June 1984, Vol. 20 No. 12, pp. 519-520. In this type of lateral conductivity modulated MOSFET, carriers accumulated in an n-type base layer must be swiftly eliminated, so as to increase the switching speed at the time of turn-off operation. If the electrons do not swiftly move from the n-type base layer to the drain layer, a pnp transistor constituted by a p-type drain layer, an n-type base layer and p-type base layer operates. As a result, a large amount of tail current flows through the conductivity modulated MOSFET, thus lengthening the turn-off interval. One of the methods of achieving a speedy turn-off operation is to shorten the life of the carriers in the n-type base layer. Although this method improves the turn-off characteristic, it undesirably raises the ON-state voltage of the element.
In the case where a conductivity modulated MOSFET is employed in an inverter circuit of a motor driving circuit, a diode is connected to the MOSFET in the backward direction and in parallel thereto, as is shown in FIG. 1. The reason for connecting the diode in this fashion is to regenerate the energy stored in the inductance component of the motor. However, such connection of the diode results in an increase in the size of the device and an increase in the manufacturing cost of the device.
To solve these problems, an anode-short structure is proposed. This structure is disclosed in, e.g., M. R. Simpson et al. "Analysis of the Lateral Insulated Gate Transistor." IEDM 85. pp 740-743. If the anode-short structure is employed, carriers accumulated in the n-type base layer are efficiently expelled through the anode-short portion during turn-off operation, so that a high-speed switching operation can be obtained. Moreover, a device employing the anode-short structure naturally incorporates a circuit equivalent to the diode shown in FIG. 1. Therefore, it is not necessary to externally connect a diode to the conductivity modulated MOSFET.
However, if the anode-short structure is employed, holes cannot be efficiently injected from the p-type drain layer into the n-type base layer. Since, therefore, the advantageous effects of conductivity modulation cannot be sufficiently obtained, the ON-state voltage of the MOSFET is undesirably raised. For satisfactory conductivity modulation, the lateral resistance of the n-type base layer located below the drain layer should be increased. To be more specific, the following measures should be taken:
(1) To widen the p.sup.+ -type drain layer extending to the anode-short portion;
(2) To reduce the impurity concentration in the n-type base layer; and
(3) To thin the n-type base layer located below the p.sup.+ -type drain layer.
However, if measure (1) is taken, the element area of the conductivity modulated MOSFET will be increased. Likewise, if measure (2) or (3) is taken, the breakdown voltage of the conductivity modulated MOSFET will be lowered.
As may be understood from the above, if a conventional conductivity modulated MOSFET employs an anode-short structure, the switching characteristic at the time of turn-off operation can be improved, but the ON-state voltage is inevitably raised. In order to employ the anode-short structure without raising the ON-state voltage, either an increase in the element area or a decrease in the breakdown voltage of the device is inevitable.